1. Technical Field of the Invention
The present invention relates generally to means and methods for attaching devices to each other, and attaching plates to other bodies, where the assemblies and/or the assemblies members get exposed to harsh operating or environmental conditions, such as when they get exposed to varying temperatures simultaneously or at varied times and varied temperature levels, and more particularly to cases where the various members of the assemblies have different thermal coefficients of expansion (TCE), what is referred to as TCE Mismatch.
More particularly, the present invention relates to attaching heat shielding and/or heat shield tiles and the like to the body of space vehicles or spacecrafts in general, in a way so as to prevent the premature failure, delamination or separation of such shielding or tiles.
2. Description of Related Art
Introduction:
Several months ago, NASA suffered a big loss. The space shuttle Columbia had an accident, it disintegrated and the astronauts perished.
To my knowledge, up to this date, a number of theories or speculations have been presented, but there has not been any conclusive determination as to the exact cause of the accident.
One of the theories points to the heat shield tiles, stating that one (or more?) tile(s) or “foam” slabs had dislodged, hit the wing or other parts of the space vehicle?, created a hole in the outside skin of the vehicle, raising the temperature, etc. etc.
The incident started me thinking about some ways to reduce the chances of such catastrophes from re-occurring, and I have applied some of my concepts in my latest patent applications to hopefully provide some options to the NASA people to consider using them.
Prior Art
A patent search using the keywords “heat shield” and “space vehicle” showed over 50 patents, many of them showing prior art in the general area of the technology. However, only one patent comes somewhat close to the invention in this application, but not close enough to create any infringement, in my opinion. Obviously, I will defer to the opinion of the Examiner to decide on this point.
1. U.S. Pat. No. 5,489,074 to Arnold et al, entitled Thermal Protection Device, In Particular For An Aerospace Vehicle.
The other patents in the prior art, listed below, were very helpful to me in learning more about this area of technology. I want to thank the authors/inventors of these patents and I will use some of the information in their patents to explain my invention here.    1. U.S. Pat. No. 3,920,339 to Fletcher et al. entitled “Strain Arrestor Plate For Fused Silica Tile”,    2. U.S. Pat. No. 4,124,732 to Leger entitled “Thermal Insulation Attaching Means”,    3. U.S. Pat. No. 4,151,800 to Dotts et al. entitled “Thermal Insulation Protection Means”,    4. U.S. Pat. No. 4,338,368 to Dotts et al. entitled “Attachment System For Silica Tiles”,    5. U.S. Pat. No. 4,439,968 to Dunn entitled “Pre-Stressed Thermal Protection Systems”.    6. U.S. Pat. No. 4,358,480 to Ecord et al. entitled “Method of Repairing Surface Damage To Porous Refractory Substrates”,    7. U.S. Pat. No. 4,706,912 to Perry entitled “Structural External Insulation For Hypersonic Missiles”,From these patents, I learned that, at different times, some parts of the spacecrafts can reach low temperatures as low as −270° F., and as high in excess of 2,300° F., or can reach up to 3,000° F. Some parts reach only 2,800° F., while other parts reach only 1,200° F., or even to 700° F. only. All these various temps are depending on the location of the part on the body of the spacecraft. So ideally, the heat shield protection has to be “tailored” to suit the specific location on the spacecraft. For this reason, I have provided the options of having either one single layer of shielding, or multiple layers of shielding, depending on the need.
Also, the shape or contour of the spacecraft varies along its outside surface, and I have provided solutions for that too.
Definitions:
I will use the words “shuttle” or “vehicle” or “spacecraft”, interchangeably, to represent/designate any space vehicle of this kind, where high amount of heat is generated during reentry from space into the earth atmosphere, or the like, i.e. any body in general that needs to have a heat shield, and where some layer(s) of heat insulation material(s) are required to be mounted on the outside of the vehicle body or the like, to act as a heat shield and to protect the insides of the vehicle from such a high level of heat and/or temp.
Tile will stand for any individual section or segment of the heat shield material, used mainly to prevent large amount of heat or cold to transfer from the outside of the space vehicle, inwards towards the vehicle body and its contents. The definition includes also any “foam” or “foam slab” or the like.
Joint will stand for any combination, where a tile is attached on to the vehicle body, the joint being considered the combination of the tile, the section or area of the vehicle body corresponding to the area of the tile and to the means of holding the tile to that area of the vehicle body, such as glue or adhesive etc. My understanding is that the tiles are simply “glued” on to the vehicle body.
Adhesive, and/or glue will stand for any method used presently to attach the heat shield tiles to the body of the space vehicle.
TCE will stand for “Temperature Coefficient of Expansion”
TR will stand for “Tie-Rod”
Each TR has two ends and I will designate them as follows. The lower end of any TR, i.e. the end near the vehicle body, will be called the “Vehicle End” or “VE”. The other end of any TR, i.e. the end near the tile, will be called the “Tile End” or “TE”. See also “Definitions”.
VE will stand for “Vehicle End” of a Tie-Rod, i.e. the end of the tie rod that is closest to the body of the vehicle.
TE will stand for “Tile End” of a Tie-Rod, i.e. the end of the tie rod that is closest to the heat shield tile.
wrt will stand for “with respect to”
L is the original length of a body under consideration,
Δt is the temperature change (increase or decrease) of the body,
k is the TCE of the material of the body,
ΔL is the change (increase or decrease) in the length L of the body, due to the given temp change.
Temp will stand for temperature.
Anchor TR is the TR that would be considered to be holding the tile more rigidly in place wrt to the rest of the members in the arrangement. Usually, it would be preferred to place the anchor TR close to the geometric center of the tile, which could also be considered the thermal center of the tile. However, in certain situation, it would be better to place the anchor TR at a strategic corner or edge of the tile.
Deformation: When a heat shield system is exposed to thermal fluctuations and other influences, causing the heat shield system to change its dimensions relative to the body of the spacecraft, said relative dimension changes will be referred to as deformation.
The Problem:
I think that one of the problems with most conventional methods of attaching/joining such tiles to the body of the shuttle is the effect of the temperature difference between one component of the “joint” and the other components. An example of such a joint, is when a tile is glued to the body or to a support member using some kind of an adhesive. Add to this problem, the effect of any mismatch in the Temperature Coefficient of Expansion (TCE) that could exist between the components of such a joint. The fact that some adhesives that are used loose their elasticity at a specific low temperature does not help the problem either.
Let me explain.
Please see FIG. 1.
Let us consider a tile 111, which is “glued” to the shuttle/vehicle body 113, as shown in the upper portion of FIG. 1.
Let's say that at Room Temperature (RT) or rather at Ambient Temperature (AT), the tile length is T1 115 and the length of the vehicle body portion or section that corresponds to this tile length is B1 117. The tile is attached to the body by some means, such as an adhesive 119 or glue or the like. The body is also at RT or AT.
At RT/AT, both parts have the same length, i.e. T1=B1.
The tile has the proper shape to match the shape of the body at this location, whether straight or curved, and the “adhesive” is holding the tile to the body.
Now, let's look at what happens when the vehicle is going through re-entry. The temperature of the tile rises to a very high level. But the tile is supposed to protect the body underneath it by preventing the heat from penetrating through. Hence, the body temperature underneath the tile is much lower.
Let's assume that the temperature difference between the tile and the body of the vehicle is 1,000° C., although it is understood that the temp difference can be much higher.
My understanding is that the tiles are made of a ceramic material, which has an average TCE of approximately 6 ppm/° C. This means that if the tile's temperature rises by 1° C., then the length of that tile would increase by 6 units of length for every 1 million unit of length of its original length.
In other words, if the tile's original length is 1 foot, and its temp rises by 1 degree C., then the tile's length would increase by [(1/1,000,000)×6] of one foot, or by [(1/1,000,000)×6×12] of one inch. This equals 0.072 of one thousandth of an inch for this 1° C. temp rise.
Now if the tile's temp rises by 1,000° C., then the tile length would increase by [0.072×1,000] of an inch, or approx. 0.072 inch.
If the tile were twice as long, i.e. 2 feet long, then the increase in its length would be twice as much, i.e. approx. 0.144 inch.
If the temp rise were twice as high, i.e. 2,000° C., then, for the 2 feet long tile, the increase in length would again be twice as much, i.e. 0.288 of an inch, i.e. over one quarter of an inch.
The physical relation is represented by the following equation:
ΔL=L×Δt×k
Where:
L is the original length of the body,
Δt is the temperature change (increase or decrease) of the body,
k is the TCE of the material of the body,
ΔL is the change (increase or de crease) in that length L of the body, due to the given temp change or thermal dimensional change or deformation.
ΔL for the tile 111 is represented in FIG. 1 by the difference between the distances 125 and 115, which is the total of the distances 131 and 133.
Note that if the tiles were made of some other material, then the k value would be different, and could possibly be larger yet than that of ceramic. Hence, the thermal dimensional change or deformation ΔL would be larger than the ones shown above.
The purpose of all this analysis is to show that the expansion of the tiles is fairly large for those high temp rises.
If the tile is attached to a substrate that remains at room temperature, because the tile is doing a good job insulating the substrate, then the difference in length between the tile and the substrate will be equal to the numbers shown above. But if, say, the substrate's temperature rises is about one half of the temp difference between the tile and the substrate, and if the TCE of the substrate is identical to the TCE of the tile, then the length difference would be one half of the numbers shown above.
However, if the TCE of the substrate is different than that of the tile, then we need to repeat the above calculations to find the changes in the substrate itself, and then determine the different in the lengths of the tile and the substrate.
Now, if we consider that the “adhesion” is the only means that is holding the tiles to the substrate, which is the body of the shuttle, then we can expect that, under these harsh temp conditions, the adhesive could break down, especially at the edges of the tiles, where the expansion is largest. Eventually, the tile could separate from the adhesive and/or the body of the shuttle. The result is that the tile would delaminate, especially at the edges of the tile. The delamination could propagate towards the center of the tile, and the end result could be that the tile could become loose and could fall off, if the rest of the adhesive is not strong enough to hold the tile down in place. This is more apt to occur, especially after some time or some repeated heating and cooling, such as after several “re-entries”.
Of course if the adhesive degrade when its temp gets below its glass transition, then the problem become much worse yet.